I don't think age is the biggest factor, it would be level on knowledge and maturity. If I have a young child, but they know the ins and outs of caring for a fish, and is mature enough to follow through, then I wouldn't have a problem with them having one.
There are plenty of teens/adults out there today that I wouldn't trust animals or fish with because they just wouldn't put enough education/time/effort behind it.
I hope that helps!

It depends on maturity, and how much your sister is going to love that betta. If she's not into fish, getting wet during water changes, having to remember to feed him, etc, she's not going to want to do so. Also, sometimes, as much as I hate to say so, once/if the novelty of having a pet fish wears off, kids in general start to 'forget' about their pets

I don't think it's that simple. Age really is just a frame of mind, though I agree about water changes being difficult for a young child. It's kind of like saying "You shouldn't date or marry until such-and-such an age". While with that one I agree that there are some definate lines, maturity is way different from person to person.

All these comments about age vs. maturity are awesome. I wasn't really thinking about that when I made this post. So here is a new question. To own a betta fish, does someone have to be super mature, pretty mature, or just relatively mature?

And to answer your question mature is mature there is no middle to maturity if your pretty,relatively,super then your not mature! That's the whole point just being responsible. I'm sorry not trying to be mean or anything

I agree that it has more to do with maturity. I'm in college and I know 20 year olds not mature enough to own a fish XD

But in general, I wouldn't get a child a pet of their own before age 8, but I would encourage a child from age 2 to help take care of the family pets. This way they would be responsible and mature enough to take care of their own pet when the time comes.

Once again, in general (there are exceptions!), I don't think most kids under 8 poses the maturity to take care of a pet by themselves. And all kids should be supervised by their parents anyways to make sure that they are taking proper care of their animals, for the animal's sake.

And uh, I would say a child would need to be pretty mature so that they don't skimp on water changes, which is very important but can be a pain in the butt to do! :)